1. Field of the Invention
This invention relates to determining the Seebeck coefficient for thermoelectric materials. Particularly, this invention relates to scanning measurement of the Seebeck coefficient of thermoelectric materials to reveal local variations and inhomogeneities.
2. Description of the Related Art
An applied temperature difference across a material causes charged carriers in the material (electrons or holes) to diffuse from the hot side to the cold side. Mobile charged carriers migrating from the hot to the cold side leave behind their oppositely charged and immobile nuclei, resulting in a thermoelectric voltage across the material. The term, “thermoelectric,” refers to the fact that the voltage is created by a temperature difference. Since a separation of charges also yields an electric field, the buildup of charged carriers on the cold side eventually ceases at some maximum value for a given temperature difference as there exists an equal amount of charged carriers drifting back to the hot side as a result of the electric field equilibrium. An increase in the temperature difference can result in more charge carriers on the cold side and thus yield an increase in the thermoelectric voltage.
The Seebeck coefficient (or thermopower) is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across a given material. The Seebeck coefficient has units of volts per degrees kelvin.
                    S        =                              Δ            ⁢                                                  ⁢            V                                Δ            ⁢                                                  ⁢            T                                              (        1        )            The Seebeck coefficient, S, depends on a material's temperature, and crystal structure. Typically, metals have small thermopowers because most have half-filled bands, including both electrons and holes. Electrons (negative charges) and holes (positive charges) both contribute to the induced thermoelectric voltage thus tending cancel their contributions to that voltage, resulting in a low net voltage. In contrast, semiconductors can be doped with an excess amount of electrons or holes and therefore can have large positive or negative values of the thermopower depending on the charge of the excess carriers. The sign of the thermopower indicates which charged carriers dominate the electric transport in both metals and semiconductors.
Accurate measurement of the Seebeck coefficient is critical for the performance assessment of thermoelectric materials. The history and challenges of Seebeck coefficient measurement has been recently reviewed. See e.g. J. Martin, T. Tritt, and C. Uher, J. Appl. Phys. 108, 121101 (2010), which is incorporated by reference herein.
The process development for large scale, homogeneous thermoelectric materials has become increasingly important due to expanding thermoelectric device product demands in various markets including the automobile industry and Peltier cooling. Recently, a number of stable materials with relatively high ZT (˜1.5), such as sodium doped PbTe, have been developed. In order to bring new types of thermoelectric materials into commercial products, the scale-up of fabrication technology is necessary.
Typical bulk prototype materials are small sized samples with sizes of about 10 mm×10 mm×1 mm. A fabricated device generally contains a large number of thermoelectric material pieces, shaped into columns by cutting a large bulk material piece. Materials fabrication techniques including the Bridgman method, Czochralski, zone-melting, variations of hot-press and Spark Plasma Sintering are often utilized to fabricate a large bulk During these processes, unintentional phase separations, grain orientation misalignments, and vacancy sites can be introduced anywhere in the bulk. This often creates local variations in a material composition thus variations in the thermoelectric parameters. For the mass production of devices, this should be avoided because each piece will have different properties, depending on where the piece was cut out from. Therefore, monitoring homogeneity of the material becomes an important part of the process optimization technique. This can be realized by detecting spatial variations of the Seebeck coefficient using apparatus such as a scanning Seebeck coefficient measurement system. This type of system has been explored and developed into production by companies such as Ulvac-Riko Inc. and PANCO GmbH. A typical setup employs a heated scanning probe (‘hot probe’) to create the temperature gradient within a sample that is necessary for the measurement.
For example, Platzek et al., “Potential-Seebeck-Microprobe (PSM): Measuring the Spatial Resolution of the Seebeck Coefficient and the Electric Potential,” 2005 International Conference on Thermoelectrics, which is incorporated by reference herein, discloses a scanning Seebeck Microprobe has been combined with the measurement of the electric potential along the surface of semiconducting or metallic material. A heated probe tip is placed onto the surface of the sample under investigation, measuring the Seebeck coefficient. Using a specially designed sample holder, an AC current can be applied to the specimen, allowing for the detection of the voltage drop between one current contact and the travelling probe tip. This voltage is proportional to the electrical conductivity at the tip position. With this technique a spatially resolved imaging of the Seebeck coefficient as well as the electrical conductivity can be performed. Furthermore the electrical contact resistance between different materials becomes visible, e.g., in segmented thermoelectric or other devices.
In view of the foregoing, there is a need in the art for improved apparatuses and methods for accurately measuring the Seebeck coefficient of thermoelectric materials. There is particularly a need for such apparatuses and methods to detect inhomogeneity in material compositions and defects throughout the thermoelectric materials. Further, there is a need for such apparatuses and methods to be simple, non-destructive, efficient, fast and affordable. There is also a need for such systems and methods to be suitable for thin film material samples. These and other needs are met by embodiments of the present invention as detailed hereafter.